Investment casting compositions, molds, and related methods

ABSTRACT

Provided are slurry compositions and methods for producing investment casting molds. These compositions include a refractory material, a binder, a solvent, and a thixotropic agent comprising a polymer emulsion. Implementation of these slurry compositions enables a reduction in the number of backup layers in an investment casting shell, while retaining similar viscosity characteristics and similar or greater strength characteristics in the finished investment mold.

FIELD OF THE INVENTION

Provided are compositions, along with related articles and methods, useful in investment casting. The compositions, more particularly, include additives that facilitate the making of an investment casting mold.

BACKGROUND

Investment casting, sometimes referred to as a “lost wax” process, is a well known method of manufacturing components having intricate and complex shapes. This process is used in diverse large- and small-scale applications, ranging from the manufacture of superalloy gas turbine engine components to tiny customized orthodontic appliances.

An investment casting process typically begins with the preparation of a sacrificial wax pattern having a size and shape similar to that of the device to be manufactured. This wax pattern can be made by molding, a rapid prototyping process, or any other method. The pattern then undergoes a shelling process in which it is sequentially dipped into tanks containing coating materials, typically ceramic slurries. Each layer of coating material is given time to dry before the next dip. Additionally, dry refractory granules, or stucco, can be applied between dips to enhance the structural integrity of the shell. This process is repeated over and over to gradually build up a shell having multiple ceramic layers.

After the shell is thus formed, the pattern is then heated, typically using a flash furnace or steam autoclave, to melt the wax and allow it to be extracted from the mold. The end result is a mold with a hollow cavity faithfully reproducing the shape of the pattern. At this point, the mold can be further strengthened by firing. A molten metal alloy can then be introduced into the mold cavity to cast the desired part. Finally, after the alloy has been sufficiently cooled, the mold can be mechanically or chemically disintegrated to separate the cast pan from the mold.

In conventional investment casting methods, the finished shell contains six or more layers, each of which could include two or more sub-layers of slurry or stucco. The first layer, known as a prime coat, is applied directly to the wax pattern. The prime coat often includes both a refractory slurry and a refractory stucco. The next layer, known as the intermediate coat, is applied over the prime coat and also includes a refractory slurry and a refractory stucco. Following application of the prime and intermediate coats, three or more backup coats are generally applied to build up the thickness of the shell. Each backup coat also commonly includes a refractory slurry and a refractory stucco. In many cases, a final seal coat is then applied over the final backup coat to prevent stucco from coming loose from the shell during further processing of the shell.

SUMMARY

Creating the aforementioned layers of the shell involves a substantial amount of time. Substantial amounts of time are involved not only in the dipping process used to apply each of the constituent slurry and/or stucco layers, but also the drying steps that follow the coating of each major layer. The large number of steps in the manufacturing process also heightens the overall risk of inadvertently inducing a defect or causing damage to the shell.

It was discovered that incorporating into a slurry composition a thixotropic agent derived from a polymer emulsion, such as an acrylic polymer emulsion, can provide a dramatic and remarkable increase in yield stress of the slurry when disposed on the investment pattern. This enables the pattern to retain a much thicker slurry layer than was previously possible using conventional investment casting slurries and additives, even those containing thixotropic agents. Using an investment slurry thus modified, it is possible to reduce the number of backup layers in an investment casting shell from four to one, while preserving acceptable slurry viscosities, conformability, and achieving similar or greater strength characteristics in the final investment mold after firing.

The aforementioned slurry compositions were further observed to resist settling over long periods of time, thereby providing a “shippable” slurry that can be mixed in advance by a manufacturer prior to delivery to the end user. Advantageously, slurry compositions could be prepared on a large scale and under precisely controlled conditions to achieve more predictable and consistent slurry compositions. Such consistency is of great value to the end user, because variation in slurry composition is known to cause shell performance issues and increase scrap rate. Both effects can adversely impact the fidelity of the final manufactured product.

In one aspect, a slurry composition for investment casting is provided. The slurry composition comprises: a refractory material; a hinder; a solvent; and a thixotropic agent comprising a polymer emulsion.

In another aspect, a method of making an investment casting mold is provided. The method comprises: coating a sacrificial pattern with a prime layer comprising a first refractory slurry and a first refractory stucco; at least partially hardening the prime layer; coating the prime layer with an intermediate layer comprising a second refractory slurry and a second refractory stucco; at least partially hardening the intermediate layer; coating the intermediate layer with a backup layer comprising a thixotropic agent, the thixotropic agent including a polymer emulsion; and at least partially hardening the backup layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a multilayered investment casting mold according to a prior art embodiment.

FIG. 2 is an enlarged fragmentary cross-sectional view of an inset portion of the investment casting mold of FIG. 1.

FIGS. 3-5 are cross-sectional views of multilayered investment casting molds according to various exemplary embodiments of the invention.

FIG. 6 is a plot of experimental data showing slurry viscosity as a function of shear rate.

FIG. 7 is a plot of experimental data showing slurry shear stress as a function of shear rate.

DEFINITIONS

As used herein:

“refractory” refers to a heat-resistant ceramic material;

“slurry” refers to a fluid mixture of a solid grain with a liquid;

“stucco” refers to a solid grain having a particle size usually not coarser than a U.S. sieve 30 mesh screen;

“thixotropic” refers to a shear-thinning property, where a gel or liquid becomes less viscous when it is shaken, agitated, or otherwise stressed;

“wax” refers to a polymeric substance capable of melting at low temperatures to yield a low viscosity liquid; and

“zircon” refers to zirconium silicate, having the chemical formula ZrSiO₄.

DETAILED DESCRIPTION

As used herein, the terms “preferred” and “preferably” refer to embodiments described herein that may afford certain benefits under certain circumstances. Other embodiments may also be preferred, under the same or other circumstances. Further, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” or “the” component may include one or more of the components and equivalents thereof known to those skilled in the art. Further, the term “and/or” means one or all of the listed elements or a combination of any two or more of the listed elements.

It is noted that the term “comprises” and variations thereof do not have a limiting meaning where these terms appear in the accompanying description. Moreover, “a,” “an,” “the,” “at least one.” and “one or more” are used interchangeably herein.

Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.

The present disclosure describes, by way of illustration and example, slurry compositions used to produce investment casting molds. The illustrated patterns and associated sprues are exemplary, not drawn to scale, and may differ widely in size and shape depending on the application at hand. It is further understood that the refractory materials, solvents, and binders herein described are exemplary and may be substituted or modified according to the knowledge of one skilled in the art.

While the compositions and related methods described herein enable one of skill in the art to make and use investment casting molds with certain advantageous properties, it is appreciated that these compositions and methods could be further combined with additives or enhancements not examined here. For example, slurry compositions could further include gaseous or solvent-based gelling agents, chemically treated refractory materials, and slurry binder systems that interact with one another.

FIG. 1 shows a conventional investment casting mold shown in cross-section and designated by the numeral 100. In this figure, the mold 100 is shown encapsulating a substantial portion of a sacrificial pattern 102, which has a tree-like structure with a centrally located trunk 103 and a plurality of branches 105 extending outwardly from the trunk 103. The pattern 102 is exemplary and there are no particular restrictions on its size or shape.

In a preferred embodiment, the pattern 102 is made from wax, polymer resin, or other suitable pattern material capable of being subsequently melted, vaporized, burned or dissolved to leave behind, with minimal residue, a cavity conforming to the exterior contours of the pattern 102.

As shown, the mold 100 includes a series of successive layers built up by dipping the pattern 102 into containers of refractory slurry. After withdrawing the pattern 102 alter each dip, excess slurry/stucco is allowed to drain off. Optionally, the pattern 102 is manipulated by hand or mechanically to promote uniform coverage. Refractory granules, or stucco, are then applied to the wet slurry coating. Here, the combination of slurry and stucco comprises a single major layer, which then is allowed to dry and at least partially harden before the next coat is applied. By repeating this process, the walls of the mold 100 are progressively built up, layer upon layer, until the overall mold 100 has the strength to withstand the physical handling forces induced by metal casting. Beginning from the innermost layer and ending with the outermost layer, mold 100 includes a prime layer 104, an intermediate layer 110, a first backup layer 116, a second backup layer 122, a third backup layer 128, and a seal layer 134.

While the mold 100 of FIG. 1 represents a six-layered construction, additional or fewer layers may also be used depending on the nature of the application. For example, factors such as the molten metal head pressure and the size of the casting to be poured from the final mold can influence the number of backup layers used. Common commercial investment casting shells often use four backup layers.

Each of the six layers enumerated above are described in further detail in reference to the inset, FIG. 2. The prime layer 104 is an innermost layer extending across and contacting the pattern 102. The prime layer 104 is intended to come into direct contact with molten metal after the finished mold 100 has been de-waxed and fired. As shown, the prime layer 104 includes two sub-layers—an inner layer of refractory slurry 106 and an outer layer of refractory stucco 108. In the mold 100 depicted in FIG. 2, both the refractory slurry 106 and refractory stucco 108 include zircon particles (shown here as round particles) although this need not be the case. In certain embodiments, one or more additional prime layers may be used. This may be the case, for example, where there is no intermediate slurry layer capability.

Referring again to FIG. 2, the intermediate layer 110, and successive backup layers 116, 122, 128 also include two sub-layers each, a layer of refractory slurry 112, 118, 124, 130 and an adjacent layer of refractory stucco 114, 120, 126, 132, respectively. The refractory slurry for these layers may include a fused silica, alumino-silicate, zircon, aluminum oxide or a mixture thereof. Similarly, the refractory stucco (represented in the figures as jagged-edged particles) may also include a fused silica, alumino-silicate, zircon, aluminum oxide, or mixture thereof. The stucco can be applied either by sprinkling it onto a freshly coated slurry by hand or by rainfall sander, or by immersion into of a fluidized bed of stucco. In some embodiments, the size of the stucco particles generally increases from the inside to the outside of the mold 100.

Optionally and as shown, a seal layer 134 is located on the outermost periphery of the mold 100. The seal layer 134 serves the purpose of preventing stucco from the backup layer 128 from coming loose during subsequent processing of the finished mold 100 and can have a composition identical or similar to that of the intermediate or backup slurries. In exemplary embodiments, the seal layer 134 contains a fused silica, alumino-silicate, zircon, aluminum oxide, or a mixture thereof.

In an exemplary method, the resulting structure as shown in FIGS. 1 and 2 can then be fully dried and heated to melt the pattern 102 and remove the pattern 102 from the finished investment casting mold 100. To add greater strength, the finished mold 100 can be fired in a curing oven at temperatures of about 980 degrees Celsius.

An improved investment casting mold 200 according to one exemplary embodiment is shown in FIG. 3. The mold 200 shares some characteristics of the mold 100. For example, like mold 100, the mold 200 includes a prime layer 204 disposed on a wax pattern 202 and an intermediate layer 210 disposed on the prime layer 204. The pattern 202, prime layer 204, and intermediate layer 210 generally share the aforementioned features, options, and advantages described with respect to the mold 100. Here, the prime layer 204 includes an inner coating of zircon-containing slurry 206 followed by an outer layer of zircon stucco 208. The intermediate layer 210, in the illustrated embodiment, includes an inner coating of refractory slurry 212 and an outer layer of refractory stucco 214. The intermediate slurry layer 210 may also contain a zircon refractory.

Referring again to FIG. 3, a single backup layer 240 is disposed on the intermediate layer 210. As shown, the backup layer 240 has a spatial thickness considerably greater than either of the prime or intermediate layers 204, 210. Advantageously, and as shown, the backup layer 240 can fill in open undercuts and cavities presented by the branches of the pattern 202, thereby simplifying subsequent coating processes. As a further major benefit, the configuration of the mold 200 eliminates the need for multiple backup layers in common investment casting applications. The backup layer 240, as shown, includes an inner coating of a refractory slurry 242 followed by a layer of refractory stucco 244. Finally, a seal layer 234 is disposed over the backup layer 240 whereby the two layers 234, 240 directly contact each other. The seal layer 234, which serves the same purposes as those of the seal layer 134, can also be omitted if desired.

In the above method, each slurry layer is optionally disposed on the pattern or underlying layer using a dipping process. When a dipping process is used, it is advantageous for the slurry to have a sufficient viscosity to be retained on the pattern or underlying layer over an acceptable working time, yet also having sufficient flowability to fill essentially all of the voids in the dipped assembly to preserve high fidelity in the mold shape. Acceptable working times generally range from about 12 seconds to about 60 seconds. The required working time for this slurry will depend upon the process and foundry, but generally is the time required for the slurry to stop draining and then be moved from above the slurry pot into the stucco application area. Using prior art methods, this time period is on the order of 2-3 minutes. These competing properties can be simultaneously achieved using the investment casting molds and methods described below.

In exemplary embodiments, the investment casting mold 200 is fabricated using methods of layer-by-layer construction analogous to those used to fabricate the investment casting mold 100, but with certain deviations as noted below. Generally, departures from the prior art methods include differences in the composition of the refractory slurry used for the backup layer(s) and, advantageously, reduction in the number of processing steps required to produce the finished investment casting mold 200.

The refractory slurry 242 has a composition that includes a refractory material, a binder, a solvent, and a thixotropic agent comprising a polymer emulsion.

The refractory material, or refractory flour or powder, is a first major component of the refractory slurry 242. Refractory powders commonly used in the investment casting industry are zircon (ZrSiO₄), silica (SiO₂), both fused and quartz, alumina (Al₂O₃), zirconia (ZrO₂), and alumino-silicate (various combinations of Al₂O₃ and SiO₂, commonly fired at high temperatures). Preferred refractory materials usable in the slurry 242 and/or stucco 244 include fused silica, alumino-silicate, zircon, aluminum oxide and mixtures thereof. Although not critical, the refractory powder can have a wide particle size distribution, including sizes as large as 30 mesh along with sub-micron particle sizes.

The binder is a second major component of the refractory slurry 242. For the purposes described herein, the binder may include a refractory binder, an organic binder, or a combination of both. Refractory binders that may be contained in the refractory slurry 242 include a variety of ceramic materials, including silicates, alkali metal silicates, silica sols, aluminum oxychloride, aluminum phosphate, gypsum-silica mixes, cements, and mixtures thereof. A particularly preferred refractory binder is colloidal silica. Organic binders can be thermally decomposable and include polyvinyl alcohol, polyvinyl butyral, methyl cellulose, carboxymethyl cellulose, ethyl cellulose, and mixtures thereof. Exemplary binders are described, for example, in U.S. Pat. No. 3,165,799 (Watts), U.S. Pat. No. 3,903,950 (Lirones), U.S. Pat. No. 5,021,213 (Kato et al.), and U.S. Pat. No. 6,020,415 (Guerra). Alternatively, the organic binder could include a mixture of colloidal sol and at least one acrylic latex polymer. The colloidal sol could be, for example, a silica sol, zirconia sol, alumina sol, or yttria sol, while the latex polymer could be an acrylic latex polymer, acrylic polymer, styrene-butadiene latex polymer, or mixture thereof.

The solvent is generally the same as the liquid dispersant used for the binder. In the presently exemplary embodiments, water is the preferred solvent. Many other solvents are available, however, including other polar solvents such as mineral acids, alcohols such as methanol, ethanol, isopropanol, and butanol, glycols and glycol ethers, and mixtures thereof. Commercial binders are often provided in solution form, so the step of adding of a separate solvent may not be necessary.

The composition of the refractory slurry 242 further includes a thixotropic agent (or shear-thinning agent) that is based on a polymer emulsion. In a preferred embodiment, the polymer emulsion is an acrylic polymer emulsion. More preferably, the polymer emulsion is an acrylic polymer emulsion in water.

Polymers suitable for this application may be prepared using any of a number of different synthetic routes. Alkali-swellable polymers, for example, are synthesized by copolymerizing different monomers, where at least one monomer contains a carboxyl (—C(OOH) functional group. These polymers may have a structure that is linear, branched or crosslinked to form a networked structure. Use of these polymers as thickening agents is described, for example, in U.S. Pat. No. 4,226,754 (Whitton et al), which discloses a polymer made by reacting an ester of methacrylic acid, methacrylic acid and a vinyl ester of a saturated aliphatic carboxylic acid. These thickeners are often referred to as alkali-swellable thickeners because the carboxylic acid groups are sufficient to render the polymer water-soluble when neutralized with a suitable base.

In other preferred embodiments, the slurry composition includes hydrophobic entities covalently bonded to the polymeric backbone. For example, polymers can be formed by reacting an ethylenically unsaturated carboxylic acid monomer, a nonionic vinyl monomer, and a vinyl surfactant ester such as an alkylphenoxypoly (ethyleneoxy) ethyl acrylate terminated on one end with an alkyl phenyl group. Another example derives from a reaction product of an unsaturated carboxylic acid, alkyl (meth)acrylate, and an ester containing an alkyl phenyl group, where the alkyl group has 8 to 20 carbon atoms. These water-soluble polymers modified with hydrophobic moieties are described in U.S. Pat. No. 4,384,096 (Sonnabend) and U.S. Pat. No. 4,138,381 (Chang et al).

In some embodiments, the slurry composition includes an acrylic emulsion copolymer is prepared using emulsion copolymerization of monomers falling within three of four classes of monomers, namely (meth)acrylic acid, alkyl (meth)acrylate, an ethoxylated ester of (meth)acrylic acid having a hydrophobic group and, optionally, a polyethylenically unsaturated monomer. In still other embodiments, the slurry composition includes an emulsion copolymer based on the reaction product of monomers including methacrylic acid, ethyl acrylate, optionally a defined copolymerizable ethylenically unsaturated monomer, and a small weight percent of a polyethylenically unsaturated monomer. Advantageously, a wide range of surfactants can enhance the thickening effect on the composition when added to an aqueous system containing the copolymer when the emulsion copolymer is neutralized. The aforementioned copolymers are further described in European Patent No. 13,836 (Chang et al.) and U.S. Pat. No. 4,421,902 (Chang et al.).

In yet another embodiment, an alkali-swellable copolymer is synthesized as the reaction product of an ethylenically unsaturated carboxylic acid, a surface-active unsaturated ester, methacrylic acid esters or acrylic acid esters of aliphatic alcohols, and optionally one or more other ethylenically unsaturated comonomers, polyethylenically unsaturated compounds, and molecular weight regulators. The surface-active ester is terminated at one end with an aliphatic radical, which may be linear or branched, a mono-, di- or tri-alkyl phenyl radical with alkyl groups of 4 to 12 carbon atoms, or a block-copolymeric radical. On partial or complete neutralization, the copolymer becomes water-soluble or colloidally dispersible, and can be used as a thickening agent. These copolymers are also described in U.S. Pat. No. 4,668,410 (Engel et al.).

One particularly advantageous thixotropic agent usable in the refractory slurry 242 is a polymer emulsion based on hydrophobically modified ester of methacrylic acid available from Elementis Specialties in Hightstown, N.J. under the tradename RHEOLATE. Methods of making such polymer emulsions are described in detail, for example, in U.S. Pat. No. 6,069,217 (Nae et al.).

Another advantageous thixotropic agent, available from the same source and under the same tradename, is based on an aqueous hydrophobically modified alkali-soluble emulsion derived from an acrylic polymer and having about 30% solids by weight. Typically, this acrylic emulsion has a pH value of less than about 5.

The polymer emulsion is preferably present in an amount that increases the yield stress of the refractory slurry to an extent that enables use of only a single backup layer while preserving strength in the investment casting mold. In some embodiments, the polymer emulsion is present in an amount of at least 0.02 weight percent, at least 0.03 weight percent, at least 0.05 weight percent, at least 0.06 weight percent, or at least 0.07 weight percent, based on the overall weight of the composition. In some embodiments, the polymer emulsion is present in an amount of at most 1 weight percent, at most 0.9 weight percent, at most 0.8 weight percent, at most 0.75 weight percent, or at most 0.7 weight percent, based on the overall weight of the composition.

Advantageously, using a polymer emulsion as a thixotropic agent allows the refractory slurry to be operated within a shear stress regime that is much lower than that of the prior art while achieving a similar working viscosity for investment casting. In some embodiments, the slurry composition displays a working viscosity of about 20 poise when subjected to a shear stress of at least 1 dyne per square centimeter, of at least 5 dynes per square centimeter, of at least 10 dynes per square centimeter, of at least 20 dynes per square centimeter, of at least 50 dynes per square centimeter, at least 100 dynes per square centimeter, at least 200 dynes per square centimeter, or at least 400 dynes per square centimeter, as measured using the method described in the Examples.

In some embodiments, the same composition displays a working viscosity of about 20 poise when subjected to a yield stress in shear of at most 1000 dynes per square centimeter, at most 950 dynes per square centimeter, at most 900 dynes per square centimeter, at most 850 dynes per square centimeter, or at most 800 dynes per square centimeter.

Investment casting shells generally have large porosity as a result of the stuccoing process, which can adversely affect strength. For the strength to be deemed adequate for a given application, it must be capable of withstanding potentially high internal pressure and thermal stress, especially during the de-waxing process and when pouring metal into the free standing ceramic shell. Cracking can occur when the stress on the mold is greater than the modulus of rupture of the mold material. In some embodiments, the investment casting mold has non-fired modulus of rupture of at least 150 psi, at least 175, at least 200 psi, at least 225, or at least 250 psi, after being fully hardened. In some embodiments, the investment casting mold has non-fired modulus of rupture of at most 750 psi, at most 735, at most 725 psi, at most 710, or at most 700 psi, after being fully hardened.

In some embodiments, the composition of the refractory slurry 242 further includes an aluminum phyllosilicate clay. In some embodiments, the aluminum phyllosilicate clay is present in an amount ranging from a weight ratio of at least 1:15, at least 1:10, at least 1:8, at least 1:7, or at least 1:6, relative to that of the polymer emulsion. In some embodiments, the aluminum phyllosilicate clay is present in an amount ranging from a weight ratio of at most 6:1, at most 5:1, or at most 4:1, relative to that of the polymer emulsion.

Combining a thixotropic thickener that includes a polymer emulsion, particularly an acrylic emulsion, with an aluminum phyllosilicate clay was observed to provide certain synergistic effects in the investment mold. For example, inclusion of both the polymer emulsion thickener and the aluminum phyllosilicate clay in the backup slurry composition was observed to substantially increase the working time of the slurry as compared with including only the aluminum phyllosilicate as thickener. When the aluminum phyllosilicate clay was used on its own, the backup slurry tended to continue to drain off of the pattern. Moreover, inclusion of both the polymer emulsion and the aluminum phyllosilicate clay was preferred over including the polymer emulsion alone because the latter tended to produce slurries that were too viscous. Such high viscosities in turn can cause delicate patterns to crack or break when inserted into the slurry. In sum, the combination of a polymer emulsion thickener and an aluminum phyllosilicate clay provided an unexpected and advantageous balance of flowability along with a long working time.

There are no particular restrictions on the overall solids present in the refractory slurry 242, but this measure should fall within a range sufficient to enable a stable colloidal suspension and yield a robust final investment casting mold 200. In some embodiments, the refractory slurry 242 has an overall solids content of at least 45 weight percent, at least 50 weight percent, or at least 55 weight percent, based on the overall weight of the composition. In some embodiments, the refractory slurry 242 has an overall solids content of at most 85 weight percent, at most 80 weight percent, or at most 75 weight percent, based on the overall weight of the composition.

Alternative embodiments are shown in FIGS. 4 and 5. FIG. 4 depicts an investment casting mold 300 according to another embodiment in which an outermost seal layer is omitted. This three-layered construction includes a prime layer 304 extending across and contacting a sacrificial pattern 302, an intermediate layer 310 extending across and contacting the prime layer 304, and a single backup layer 340 extending across and contacting the intermediate layer 310. Like in the embodiment previously described, each of the layers 304, 310, 340 includes an inner sub-layer of refractory slurry adjoining an outer sub-layer of a refractory stucco.

Absent from the mold 300 is an outermost seal layer; in FIG. 4, the layered construction ends with the refractory stucco for the backup layer 340. While sharing most of the functional properties of the mold 200, the mold 300 requires even fewer processing steps to fabricate.

FIG. 5 illustrates an investment casting mold 400 according to yet another embodiment. Compared with prior embodiments, the mold 400 is notably even further simplified in its two-layered construction. Showing merely a prime layer 404 and backup layer 440 disposed on a pattern 402, the mold 400 can advantageously be made using only two dips—one for each of layers 404, 440. Other aspects of the mold 400 and its constituent layers are essentially the same as those described with respect to the three- and four-layered embodiments above.

Ideally, an investment casting slurry composition displays a yield stress that is sufficient to prevent excessive drainage of the slurry from a pattern after the pattern is withdrawn from a bath of the slurry. This characteristic should be tempered, however, by its flowability—essentially, its ability to flow into and around complex pattern geometries, including narrow cavities, when the pattern is dipped into the slurry. The slurry compositions provided here operate in a solid-like regime at the low shear rates associated with gravity, but operate in a liquid-like regime at higher shear rates associated with dipping the pattern into a bath of the slurry. By minimizing gravity-induced drainage while simultaneously achieving good flowability in the dipping process, the provided compositions reduce the number of required dips while preserving the fidelity of the final molded product.

In some embodiments, the yield stress of the slurry composition is at least 0.2 dynes/cm², at least 0.5 dynes/cm², at least 1 dyne/cm², at least 5 dynes/cm², or at least 10 dynes/cm². In the same or alternative embodiments, the yield stress of the slurry composition can be at most 200 dynes/cm², at most 250 dynes/cm², at most 500 dynes/cm², at most 750 dynes/cm², or at most 1000 dynes/cm². Exemplary slurry compositions, at the onset of flow, can display a viscosity at the onset of flow of at least 50 poise, at least 100 poise, at least 200 poise, at least 500 poise, or at least 1000 poise. In the same or alternative embodiments, the viscosity of the slurry composition at the onset of flow can be at most 7000 poise, at most 8000 poise, at most 9000 poise, at most 10000 poise, or at most 12000 poise.

Technical features and advantages of the invention are further exemplified by, but are not necessarily limited to, the following enumerated embodiments A-AI:

A. A slurry composition for investment casting including: a refractory material; a binder; a solvent; and a thixotropic agent including a polymer emulsion. B. The composition of embodiment A, where the polymer emulsion is an aqueous emulsion. C. The composition of embodiment A or B, where the polymer emulsion includes an alkali-swellable polymer. D. The composition of embodiment C, where the alkali-swellable polymer includes a hydrophobically modified ester of methacrylic acid. E. The composition of any one of embodiments A-D, further including an aluminum phyllosilicate clay. F. The composition of embodiment E, where the aluminum phyllosilicate clay is present in an amount ranging from a weight ratio of 1:15 to 6:1 relative to the polymer emulsion. G. The composition of embodiment F, where the aluminum phyllosilicate clay is present in an amount ranging from a weight ratio of 1:8 to 4:1 relative to the polymer emulsion. H. The composition of embodiment G, where the aluminum phyllosilicate clay is present in an amount ranging from a weight ratio of 1:6 to 4:1 relative to the polymer emulsion. I. The composition of any one of embodiments A-H, where the polymer emulsion is present in an amount ranging from 0.02 weight percent to 1 weight percent, based on the overall weight of the composition. J. The composition of embodiment I, where the polymer emulsion is present in an amount ranging from 0.05 weight percent to 1 weight percent based on the overall weight of the composition. K. The composition of embodiment J, where the polymer emulsion is present in an amount ranging from 0.07 weight percent to 0.75 weight percent, based on the overall weight of the composition. L. The composition of any one of embodiments A-K, where the composition has an overall solids content ranging from 45 weight percent to 80 weight percent, based on the overall weight of the composition. M. The composition of embodiment L, where the composition has an overall solids content ranging from 45 weight percent to 75 weight percent, based on the overall weight of the composition. N. The composition of embodiment M, where the composition has an overall solids content ranging from 50 weight percent to 75 weight percent, based on the overall weight of the composition. O. The composition of any one of embodiments A-N, where the solvent is water. P. The composition of any one of embodiments A-O, where the binder includes colloidal silica. Q. The composition of any one of embodiments A-P, where the composition displays a working viscosity of about 20 poise when subjected to a yield stress ranging from 25 dynes/cm² to 1000 dynes/cm². R. The composition of embodiment Q, where the composition displays a working viscosity of 20 poise when subjected to a yield stress ranging from 25 dynes/cm² to 900 dynes/cm². S. The composition of embodiment R, where the composition displays a working viscosity of 20 Poise when subjected to a yield stress ranging from 25 dynes/cm² to 800 dynes/cm². T. The composition of any one of embodiments A-S, where the binder includes a styrene-butadiene latex. U. The composition of any one of embodiments A-S, where the binder includes a polyvinyl butyral resin. V. An investment casting mold made using the composition of any one of embodiments A-U. W. A method of making an investment casting mold including: coating a sacrificial pattern with a prime layer including a first refractory slurry and a first refractory stucco; at least partially hardening the prime layer; coating the prime layer with an intermediate layer including a second refractory slurry and a second refractory stucco; at least partially hardening the intermediate layer; coating the intermediate layer with a backup layer including a thixotropic agent, the thixotropic agent including a polymer emulsion; and at least partially hardening the backup layer. X. The method of embodiment W, where the first refractory slurry includes zircon. Y. The method of embodiment W or X, where the first refractory stucco includes zircon. Z. The method of any one of embodiments W-Y, where the second refractory slurry includes fused silica, alumina, alumino-silicate, or a mixture thereof. AA. The method of any one of embodiments W-Z, where the second refractory stucco includes fused silica, alumina, alumino-silicate, or a mixture thereof. AB. The method of any one of embodiments W-AA, further including: coating the backup layer with a seal layer including a third refractory slurry, where the seal layer directly contacts the backup layer; and at least partially hardening the seal layer. AC. The method of embodiment AB, where the third refractory slurry includes a fused silica slurry, alumino-silicate slurry, or mixture thereof. AD. The method of any one of embodiments W-AC, where the investment casting mold has a non-fired modulus of rupture ranging from 150 psi to 750 psi after fully hardened. AE. The method of embodiment AD, where the investment casting mold has a non-fired modulus of rupture ranging from 200 psi to 700 psi after fully hardened. AF. The method of embodiment AE, where the investment casting mold has a non-fired modulus of rupture ranging from 250 psi to 700 psi after fully hardened. AG. The method of any one of embodiments W-AF, further including applying heat to remove the pattern from the investment casting mold. AH. The method of any one of embodiments W-AG, where the second refractory stucco has a size ranging from U.S. Sieve 10 mesh to U.S. Sieve 100 mesh. AI. The method of any one of embodiments W-AH, where the backup layer further includes a refractory stucco having a mesh size ranging from U.S. Sieve 30 mesh to U.S. Sieve 100 mesh.

EXAMPLES Materials

“WDS II”, fused silica flour was obtained from 3M Midway, Midway, Tenn., under the trade designation “WDS II”.

“WDS 3”, fused silica flour, was obtained from 3M Midway, Midway, Tenn., under trade designation “WDS 3”.

“Min-Sil 120F”, fused silica flour, was obtained from 3M Midway, Midway, Tenn., under trade designation “Min-Sil 120F”.

“NALCO 1130”, silica sol, 30 weight % SiO₂, 8 nanometer particle size, was obtained from Nalco Chemical Company, Naperville, Ill., under trade designation “NALCO 1130”.

“NALCO 6300”, a styrene-butadiene latex polymer, 50 weight % solids, was obtained from Nalco Chemical Company, Naperville, Ill., under trade designation “NALCO 6300”.

“Minco HP”, a styrene butadiene latex polymer, 50 weight % solids, was obtained from 3M Midway, Midway, Tenn., under trade designation “Minco HP”.

“NALCO 2305”, antifoam additive containing a blend of silicones and polyglycols in a hydrocarbon solvent, was obtained from Nalco Chemical Company. Naperville, Ill., under trade designation “NALCO 2305”.

“NALCO 8815”, a wetting agent, was obtained from Nalco Chemical Company, Naperville, Ill., under trade designation “NALCO 8815”.

“BENTONE EW”, highly beneficiated, easily dispersible powdered clay thickener, was obtained from Elementis, Specialties, Inc., Hightstown, N.J., under trade designation “BENTONE EW”.

“RHEOLATE 420”, an alkali swellable thickener, was obtained from Elementis, Specialties, Inc., Hightstown, N.J., under trade designation “RHEOLATE 420”.

“RHEOLATE 288”, a highly efficient polyether polyurethane associative thickener, was obtained from Elementis, Specialties, Inc., Hightstown, N.J., under trade designation “RHEOLATE 288”.

“RHEOLATE 1”, an acrylic thickener with high thickening efficiency, was obtained from Elementis, Specialties. Inc., Hightstown, N.J., under trade designation “RHEOLATE 1”.

“RHEOLATE 278”, a highly efficient polyether polyurethane associative thickener, was obtained from Elementis, Specialties, Inc., Hightstown, N.J., under trade designation “RHEOLATE 278”.

“SOLTHIX A300”, an alkali swellable thickener, was obtained from Lubrizol Advanced Materials, Inc., Brecksville, Ohio, under trade designation “SOLTHIX A300”.

“SOLTHIX A100”, an alkali swellable thickener, was obtained from Lubrizol Advanced Materials, Inc., Brecksville, Ohio, under trade designation “SOLTHIX A100”.

“THIXATROL PLUS”, an active, seed resistant organic rheological additive, was obtained from Elementis, Specialties, Inc., Hightstown, N.J., under trade designation “THIXATROL PLUS”.

Fused silica, 50×100 mesh (finer than U.S. Sieve 50 mesh but coarser than U.S. Sieve 100 mesh), was obtained from 3M Midway, Midway, Tenn.

Fused silica, 30×50 mesh (finer than U.S. Sieve 30 mesh but coarser than U.S. Sieve 50 mesh), was obtained from 3M Midway. Midway, Tenn.

General Method for Preparing Prime Slurry, Intermediate Slurry, and Backup Slurry

Into a sufficient volume container, de-ionized (DI) water and NALCO 1130 silica sol were added. While mixing using a INDCO Model HS120T mixer (2 horsepower, 220 V, single phase motor, set at a speed of 2050 rpm), desired amounts of silica flour, additives such as polymeric binders (e.g., styrene-butadiene latex), antifoam and/or wetting agents were added and mixing was continued until all the lumps were dispersed. Finally, if desired, a rheological additive (i.e., a thixotropic agent) was added and mixing was continued, typically for less than 5 minutes.

General Method for Preparing Investment Casting Molds

Investment casting molds were made using a multi-step process. First, a wax pattern having the shape of final investment cast parts was provided. On top of the wax pattern, investment cast molds were formed by building a series of shells (i.e., layers) sequentially. In a first step, the wax pattern was coated by a “prime layer” which comprises an initial coating of prime slurry layer which was further coated with a prime stucco layer. The prime slurry layer was formed by dipping the wax pattern in the prime slurry for about 20 seconds while rotating and moving the wax pattern to maximize the uniformity of the prime slurry layer. A prime stucco layer was then deposited on the wet prime slurry layer by exposing the wax pattern with the prime slurry layer thereon to a fluidized bed of 50×100 mesh zircon particles. The wax pattern with the prime stucco layer was then dried at 21 degrees Celsius for about 2 hours. Afterwards, the wax pattern with the dried prime layer was coated with an “intermediate layer” in essentially the same manner as the prime layer except by using an intermediate slurry and stucco layers and dried. The intermediate stucco layer was formed using a fluidized bed of 50×100 mesh fused silica particles. The composition of the intermediate slurry could be the same or different than the primary slurry. The resulting pattern was then coated with one or more backup layer(s) in essentially the same manner as the primary/intermediate layer except using backup slurry and stucco layers and dried. The backup stucco layer was formed using a fluidized bed of 30×50 mesh fused silica particles. The composition of the backup slurry could be the same or different than the primary/intermediate slurry. The backup slurry layer/stucco layer building is typically repeated several times to build enough thickness with sufficient drying between each layer. Finally, the pattern with sufficiently thick backup layer(s) was coated with a seal layer by dipping it again into the backup slurry and drying. The final investment casting molds were freed of the wax pattern, fired and used for testing and/or preparing final investment cast parts.

Example and Comparative Example investment casting molds, prepared as above, were characterized in their “green” states and/or after firing.

Method for Measuring Viscosity

Viscosity and shear stress data for slurries were measured using an AR G2 stress controlled rheometer (TA Instruments, New Castle, Del.) outfitted with a 40-mm diameter parallel plate fixture. Measurements were made using a gap of 1 mm and an operating temperature of 23 degrees Celsius.

Slurries were tested using a continuous flow shear rate sweep. Tests were conducted with an ascending shear rate from 10⁻³ s⁻¹ to 100 s⁻¹, and then descending shear rate down to 10⁻¹ s⁻¹. The yield stress of each slurry was obtained by plotting shear stress as a function of total strain for ascending shear rates, identifying regimes of fluid-like and solid-like behavior along the plot, fitting a power law to each regime, then determining shear stress at the intersection point between these fits. The viscosity at the onset of flow was also determined based on the measured viscosity at the time yield stress was first reached.

Method for Determining the Bend Strength

To prepare strength testing samples, standard stainless steel bars 1 in.×0.25 in.×13 in. (2.54 cm×0.64 cm×33 cm) were covered with investment casting shells prepared from slurries used in Examples and Comparative Examples in the same manner as preparing the investment cast molds described above. Before coating with the investment casting shells, the steel bars were first coated with wax (S.C. Johnson's Paste Wax, commercially available from S.C. Johnson & Sons, Inc., Racine, Wis.). The resulting shells were separated from the steel plates and were used for bend strength testing. The strength testing of the shell samples were carried out using a Universal Test Machine (Model SSTM-1, obtained from United Test Machine of Huntington Beach, Calif.) using a cross head speed of 0.05 in. (0.13 cm) per minute along with a 2 in. (5 cm) span. The thickness of the test samples at break was measured in six locations across the break, three on either side of the break and the measurements averaged. The width was measured twice and the measurements averaged. The strength test data reported was average of 24 test samples for each Example and Comparative Example investment cast mold compositions. The strength data for Example samples were run along with the corresponding Comparative Example samples. The strength test data e.g., modulus of rupture (MOR), modulus of elasticity (MOE), and load at failure were determined. The strength testing was done in the green and fired states under variety of environmental conditions.

Method for Permeability and Burst Testing

For this test, samples were prepared by building shells using the slurries prepared according to the Examples and Comparative Examples, on polyvinylchloride (PVC), schedule 40 cold plumbing pipes. The PVC pipes had 0.75 in. (1.09 cm) inner diameter and 1.05 in. (2.77 cm) outer diameter and were 13 in. (33 cm) long. The pipes were first coated with wax (S.C. Johnson's Paste Wax). After the shells were built, the resulting samples were cut into 6 in. (15.2 cm) long sections for testing. The permeability and burst testing was done using the method described in Snyder, B. and Snow, J. “A New Combination Shell Strength and Permeability Test,” in the 51^(st) Annual Technical Meeting of the Investment Casting Institute, 2003, p. 11:1-25 (published by the Investment Casting Institute). Ten sections (i.e., samples) were tested for each Example and Comparative Example.

Comparative Example 1 (CE-1)

CE-1 investment cast mold was prepared using the general method for preparing investment casting molds described above, except that no prime layer was applied and that the CE-1 investment cast mold included five backup layers. The compositions of the intermediate, backup, and seal slurry layer used for preparing CE-1 investment cast mold was the same and the slurry was prepared as described in the general method for preparing prime slurry, intermediate slurry, and backup slurry described above by mixing 13,705 g of WDS II silica flour, 4,516 g of NALCO 130, 934 g DI water, 498 g Minco HP latex binder, and 21 g NALCO 2305 antifoam additive.

The CE-1 investment cast molds were fired at 2000° F. (1093° C.) for two hours before use.

Comparative Example 2 (CE-2)

CE-2 investment cast mold was prepared using the general method for preparing investment casting molds described above, except that no prime layer was applied and that the CE-2 investment cast mold included five backup layers. The compositions of the intermediate, backup, and seal slurry layer used for preparing CE-2 investment cast mold was the same and the slurry was prepared as described in the general method for preparing prime slurry, intermediate slurry, and backup slurry described above by mixing 13,305 g of WDS 3 silica flour, 6,259 g of NALCO 1130, 545 g NALCO 6300, 285 g DI water, 10 g NALCO 2305 antifoam additive, and 10 g NALCO 8815 wetting additive.

The CE-2 investment cast molds were fired at 2000° F. (1093° C.) for two hours before use.

Example 3 (EX-3)

EX-3 investment cast mold was prepared using the general method for preparing investment casting molds described above, except that no prime layer was applied and that the EX-3 investment cast mold included only one backup layer. The compositions of the intermediate, and seal slurry layer were same as that used for preparing CE-1 investment cast mold as described above. The slurry used for EX-3 backup layer had a unique composition that enabled a useful, single backup layer. The EX-3 backup slurry was prepared as described in the general method for preparing prime slurry, intermediate slurry, and backup slurry described above by mixing 6,750 g of Min-Sil 120F fused silica flour, 3,362 g of NALCO 1130, 476 g DI water, 164 g styrene-butadiene latex binder, and 15 g RHEOLATE 420 rheological additive (thixotrope).

The EX-3 investment cast molds were fired at 2000° F. (1093° C.) for two hours before use.

The CE-1, CE-2 and EX-3 formulations were used to prepare sufficient number of permeability, burst, and strength testing under a variety of test conditions as described below. Sample preparation and testing was carried out using the procedures described above. Test results obtained are described below.

Shell Thickness

CE-1, CE-2 and EX-3 samples prepared for strength testing and permeability testing were used to determine the thickness of the shells built for each formulation. The shell thickness data is summarized in Table 1, below.

TABLE 1 Shell thickness Strength test samples Permeability test samples 95% 2X 95% Thickness Standard error Thickness Standard error Example (cm) (cm) (cm) (cm) CE-1 0.51 0.025 0.53 0.025 CE-2 0.69 0.025 0.66 0.00 EX-3 0.51 0.025 0.48 0.051

Table 2, below, summarizes the permeability and burst test data for CE-1, CE-2, and EX-3 obtained using the method described above.

TABLE 2 Pipe Burst Test Strength test samples Maximum 95% tangential 2X 95% Permeability Standard error stress Standard error Example (cm²) (cm²) (kPa) (kPa) CE-1 3.6 × 10⁻¹⁰ 2.2 × 10⁻¹¹ 358.53 19.31 CE-2 8.9 × 10⁻¹⁰ 5.7 × 10⁻¹¹ 234.42 18.62 EX-3 6.8 × 10⁻¹⁰ 8.3 × 10⁻¹¹ 275.79 17.24

Table 3, below, summarizes the green strength test data for green CE-1, CE-2, and EX-3, obtained using the method described above.

TABLE 3 MOR/95% MOE/95% Failure Load/95% Standard error Standard error Standard error Example (MPa) (MPa) (N) CE-1 4.08/0.15 1.67/0.12 35.59/2.67 CE-2 2.36/0.17 1.06/0.23 35.14/2.22 EX-3 3.34/0.17  1.54/0.0.15 28.47/2.67

Table 4, below, summarizes the hot/wet strength test data after boiling for 15 minutes in water (e.g. “hot/wet” state). CE-1, CE-2, and EX-3 were obtained using the method described above.

TABLE 4 MOR/95% MOE/95% Failure Load/95% Standard error Standard error Standard error Example (MPa) (MPa) (N) CE-1  1.44/0.137 0.81/0.09 12.46/1.34 CE-2 0.77/0.07 0.35 0.05 11.57/0.89 EX-3 1.57/0.17 0.99/0.17 13.79/1.78

Table 5, below, summarizes the fired (after cooling to room temperature) strength test data for CE-1, CE-2, and EX-3, obtained using the method described above.

TABLE 5 MOR/95% MOE/95% Failure Load/95% Standard error Standard error Standard error Example (MPa) (MPa) (N) CE-1 1.79/0.07 0.57/0.06 14.23/089 CE-2 1.32/0.07 0.28/0.02  20.91/1.33 EX-3 1.52/0.06 0.90/0.08 11.57/.45 

Table 6, below, summarizes the fired (and tested while hot) strength test data for CE-1, CE-2, and EX-3, prepared using the method described above.

TABLE 6 MOR/95% MOE/95% Failure Load/95% Standard error Standard error Standard error Example (MPa) (MPa) (N) CE-1 9.15/0.48 4.47/0.38 74.73/5.78 CE-2 7.50/0.26 2.96/0.30 110.31/6.23  EX-3 7.50/0.39 4.26/0.32 61.83/4.45

Examples 5-22 (EX-5 to EX-22) and Comparative Example 4 (CE-4)

Comparative CE-4 and Examples EX-5 to EX-19 were prepared to demonstrate the variation of rheological properties of backup slurries with the amount and type of various rheological additives. CE-4 was prepared in the same manner as EX-3 except CE-4 did not contain any rheological additive. EX-5 to EX-22 were prepared in the same manner as EX-3 except that the rheological additive RHEOLATE 420 was replaced with various rheological additives as summarized in Table 7, below.

TABLE 7 Rheological Rheological Additive I, Amount Additive II, Amount Example (g) (g) EX-5 THIXATROL PLUS, 100 BENTONE EW, 0 EX-6 THIXATROL PLUS, 100 BENTONE EW, 7.5 EX-7 THIXATROL PLUS, 100 BENTONE EW, 15 EX-8 RHEOLATE 288, 44 BENTONE EW, 0 EX-9 RHEOLATE 288, 44 BENTONE EW, 7.5 EX-10 RHEOLATE 288, 44 BENTONE EW, 15 EX-11 RHEOLATE 278, 14 BENTONE EW, 0 EX-12 RHEOLATE 278, 14 BENTONE EW, 7.5 EX-13 RHEOLATE 278, 14 BENTONE EW, 15 EX-14 RHEOLATE 1, 10 BENTONE EW, 0 EX-15 RHEOLATE 1, 10 BENTONE EW, 7.5 EX-16 RHEOLATE 1, 10 BENTONE EW, 15 EX-17 SOLTHIX A100, 15 BENTONE EW, 0 EX-18 SOLTHIX A100, 15 BENTONE EW, 7.5 EX-19 SOLTHIX A100, 15 BENTONE EW, 15 EX-20 SOLTHIX A300, 15 BENTONE EW, 0 EX-21 SOLTHIX A300, 15 BENTONE EW, 7.5 EX-22 SOLTHIX A300, 15 BENTONE EW, 15

Examples 24-26 (EX-24 to EX-26) and Comparative Example 23 (CE-23)

EX-24 to EX-26 and CE-23 were prepared in lab using a “Magic Bullet” high intensity mixer, following the formulations shown in Table 8, below. First, the NALCO 1130 colloidal liquid, Minco HP styrene-butadiene latex, and DI water were hand mixed, and the Min-Sil 120F fused silica flour was added, and mixed by high intensity mixing for 1 minute. After this step, CE-23 was ready.

For EX-24 to EX-26, additional mixing steps were used to incorporate additional components. For EX-24 and EX-25. BENTONE EW was added to the batch, mixed lightly by hand to wet the clay, then mixed in a high-intensity mixer for 30 seconds. RHEOLATE 420 was then added to the batch and mixed at high intensity for 1 minute. All slurries were tested within 24 hours of formulation to determine their rheological properties. The samples were hand mixed prior to testing to eliminate any settling.

TABLE 8 Amount Added (g) Material EX-24 EX-25 EX-26 CE-23 NALCO 1130 100 100 100 100 Minco HP 4.81 4.81 4.81 4.81 DI Water 13.56 13.56 13.56 13.56 Min-Sil 120F 200.64 200.64 200.64 208.64 BENTONE EW 0.32 0.16 0 0 RHEOLATE 420 0.96 0.96 0.96 0

The yield stress and the viscosity at the onset of flow were obtained for each Example/Comparative Example in Table 8 above. The results are summarized in Table 9, below.

TABLE 9 Yield stress, ascending rate Viscosity, η, at onset of flow Example (dyne/cm²) (poise) EX-24 101 5600 EX-25 43.4 4600 EX-26 22.0 3700 CE-23 0.156 23

Examples 27-32 (EX-27 to EX-32) and Comparative Example 24-25 (CE-24 to CE-25

EX-27 to EX-32 investment cast molds were prepared using the general method for preparing investment casting molds described above, except as follows: For each of EX-27 to EX-32, in a first step, the wax pattern was coated by a “prime layer” which comprised an initial coating of prime slurry layer which was further coated with a prime stucco layer. The prime slurry layer was formed by dipping the wax pattern in the prime slurry (which was the same composition as the slurry used in CE-1) for about 20 seconds while rotating and moving the wax pattern to maximize the uniformity of the prime slurry layer. A prime stucco layer was then deposited on the wet prime slurry layer by exposing the wax pattern with the prime slurry layer thereon to a fluidized bed of 50×100 mesh zircon particles. The wax pattern with the prime stucco layer was then dried at 21 degrees Celsius for about 2 hours. Afterwards, the wax pattern with the dried prime layer was coated with the corresponding Example slurries followed by a stucco layer which was formed using a fluidized bed of 30×50 mesh fused silica particles. The slurries used for EX-27 to EX-32 backup layers had unique compositions that enabled a useful, single backup layer. Finally, after drying the resulting EX-27 to EX-32 molds for 18-24 hours, each mold were applied with a seal layer (also known as a cover coat). The composition of the seal layer slurry was the same as CE-1 slurry.

EX-27 slurry was prepared as described in the general method for preparing prime slurry, intermediate slurry, and backup slurry described above by mixing 13,500 g of Min-Sil 120F fused silica flour, 15 g of BENTONE EW, 6,724 g of NALCO 1130, 952 g DI water, 328 g styrene-butadiene latex binder, and 60 g RHEOLATE 475 rheological additive (thixotrope).

EX-28 slurry was the same as EX-27 slurry, except that it was aged for 1 month before use.

EX-29 slurry was same as EX-27 slurry except that it contained 30 g of BENTONE EW.

EX-30 slurry was the same as EX-29 slurry, except that it was aged for 1 month before use.

EX-31 slurry was same as EX-27 slurry except that it contained 120 g of RHEOLATE 475.

EX-32 slurry was same as EX-31 slurry except that it was aged for 1 month before use.

CE-24 and CE-25 were same as CE-1 and CE-2, respectively.

CE-24, CE-25 and EX-27 to EX-32 formulations were used to prepare sufficient number of permeability, burst, and strength test samples for testing under a variety of test conditions as described below. Sample preparation and testing was carried out using the procedures described above. Test results obtained are summarized below.

CE-24, CE-25 and EX-27 to EX-32 samples prepared for strength testing (24 samples per Example) and permeability testing (10 samples per Example) were used to determine the thickness of the shells built for each formulation. The shell thickness data is summarized in Table 10, below.

TABLE 10 Shell thickness Strength test samples Permeability test samples 95% 2X 95% Thickness Standard error Thickness Standard error Example (cm) (cm) (cm) (cm) CE-24 0.20 0.010 0.21 0.010 CE-25 0.31 0.010 0.30 0.010 EX-27 0.20 0.008 0.20 0.004 EX-28 0.20 0.012 0.19 0.006 EX-29 0.28 0.009 0.25 0.008 EX-30 0.21 0.010 0.25 0.010 EX-31 0.22 0.009 0.20 0.002 EX-32 0.23 0.009 0.21 0.010

Table 11, below, summarizes the permeability and burst test data for CE-24, CE-25 and EX-27 to EX-32 obtained using the method described above.

TABLE 11 Pipe Burst Test Strength test samples Maximum 95% tangential 2X 95% Permeability Standard error stress Standard error Example (cm²) (cm²) (kPa) (kPa) CE-24 3.6 × 10⁻¹⁰ 2.2 × 10⁻¹¹ 358 20.7 CE-25 7.6 × 10⁻¹⁰ 7.6 × 10⁻¹¹ 338 15.2 EX-27 4.7 × 10⁻¹⁰ 4.8 × 10⁻¹¹ 96.5 5.5 EX-28 5.6 × 10⁻¹⁰ 4.0 × 10⁻¹¹ 13.8 6.9 EX-29 4.6 × 10⁻¹⁰ 3.9 × 10⁻¹¹ 207 26.9 EX-30 4.9 × 10⁻¹⁰ 4.6 × 10⁻¹¹ 262 21.3 EX-31 3.6 × 10⁻¹⁰ 4.1 × 10⁻¹¹ 96.5 26.2 EX-32 3.8 × 10⁻¹⁰ 3.5 × 10⁻¹¹ 55.2 7.6

Table 12, below, summarizes the green strength test data for green CE-24, CE-25 and EX-27 to EX-32, obtained using the method described above.

TABLE 12 MOR/95% MOE/95% Failure Load/95% Standard error Standard error Standard error Example (MPa) (MPa) (N) CE-24 4.08/0.14 1.67/0.12 35.6/4.5 CE-25 3.42/0.12 0.97/0.06 80.1/4.5 EX-27 4.86/0.20 2.21/0.24 44.5/4.9 EX-28 3.19/0.21 1.28/0.28 26.7/2.7 EX-29 3.81/0.15 1.29/0.12 62.3/4.5 EX-30 3.99/0.20 1.48/0.20 17.8/4.0 EX-31 4.47/0.20 2.04/0.22 40.0/3.6 EX-32 4.59/0.25 1.48/0.12 49.0/4.5

Table 13, below, summarizes the hot/wet strength test data after boiling for 15 minutes in water (e.g. “hot/wet” state). CE-24, CE-25 and EX-27 to EX-32 were obtained using the method described above.

TABLE 13 MOR/95% MOE/95% Failure Load/95% Standard error Standard error Standard error Example (MPa) (MPa) (N) CE-24 1.44/0.13 0.81/0.09 12.5/1.34  CE-25  0.9/0.06 0.30/0.03 18.3/1.34  EX-27 1.03/0.08 0.72/0/11 8.9/0.89 EX-28 1.01/0.10 0.68/0.12 8.9/0.89 EX-29 0.94/0.10 0.70/0.13 22.2/1.3  EX-30 1.25/0.08 0.57/0.10 8.9/0.89 EX-31 1.12/0.14 0.79/0.10 8.9/1.78 EX-32 0.87/0.09 0.61/0.23 8.9/0.89

Table 14, below, summarizes the tired (after cooling to room temperature) strength test data for CE-24, CE-25 and EX-27 to EX-32, obtained using the method described above.

TABLE 14 MOR/95% MOE/95% Failure Load/95% Standard error Standard error Standard error Example (MPa) (MPa) (N) CE-24 1.79/0.07 0.57/0.06 13.4/0.00 CE-25 1.43/0.08 0.29/0.02 26.7/0.00 EX-27 1.59/0.14 0.67/0.11 13.4/0.89 EX-28 1.80/0.12 1.01/0.14 13.4/0.89 EX-29 1.59/0.13 0.50/0.08 26.7/1.78 EX-30 1.83/0.1  0.83/0.08 17.8/1.34 EX-31 1.74/0.12 0.81/0.11 17.8/1.78 EX-32 2.23/0.19 0.79/0.11 22.3/2.23

Table 15, below, summarizes the fired (and tested while hot) strength test data for CE-24, CE-25 and EX-27 to EX-32, prepared using the method described above.

TABLE 15 MOR/95% MOE/95% Failure Load/95% Standard error Standard error Standard error Example (MPa) (MPa) (N) CE-24 9.15/0.48 4.47/0.38 76/4.5 CE-25 7.37/0.40 2.14/0.25 156/8.9  EX-27 7.67/0.59 4.54/0.49 67/5.8 EX-28 8.18/0.54 4.61/0.57 67/5.8 EX-29 7.11/0.61 2.88/0.39 107/7.1  EX-30 7.29/0.34 3.72/0.48 76/4.9 EX-31 7.89/0.57 4.48/0.48 76/4.9 EX-32 8.85/0.45 4.58/0.46 93/6.7

All patents and patent applications mentioned above are hereby expressly incorporated by reference. Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the following claims and their equivalents. 

1. A slurry composition for investment casting comprising: a refractory material; a binder; a solvent; and a thixotropic agent comprising a polymer emulsion.
 2. The composition of claim 1, wherein the polymer emulsion is an aqueous emulsion.
 3. The composition of claim 1, wherein the polymer emulsion comprises an alkali-swellable polymer.
 4. The composition of claim 3, wherein the alkali-swellable polymer comprises a hydrophobically modified ester of methacrylic acid.
 5. The composition of claim 1, further comprising an aluminum phyllosilicate clay.
 6. The composition of claim 5, wherein the aluminum phyllosilicate clay is present in an amount ranging from a weight ratio of 1:6 to 4:1 relative to the polymer emulsion.
 7. The composition of claim 1, wherein the polymer emulsion is present in an amount ranging from 0.07 weight percent to 0.75 weight percent, based on the overall weight of the composition.
 8. The composition of claim 1, wherein the composition has an overall solids content ranging from 50 weight percent to 75 weight percent, based on the overall weight of the composition.
 9. The composition of claim 1, wherein the binder comprises colloidal silica.
 10. The composition of claim 1, wherein the composition displays a working viscosity of 20 Poise when subjected to a yield stress ranging from 25 dynes/cm² to 800 dynes/cm².
 11. The composition of claim 1, wherein the binder comprises a styrene-butadiene latex.
 12. The composition of claim 1, wherein the binder comprises a polyvinyl butyral resin.
 13. An investment casting mold made using the composition of claim
 1. 14. A method of making an investment casting mold comprising: coating a sacrificial pattern with a prime layer comprising a first refractory slurry and a first refractory stucco; at least partially hardening the prime layer; coating the prime layer with an intermediate layer comprising a second refractory slurry and a second refractory stucco; at least partially hardening the intermediate layer; coating the intermediate layer with a backup layer comprising a thixotropic agent, the thixotropic agent including a polymer emulsion; and at least partially hardening the backup layer.
 15. The method of claim 14, wherein the investment casting mold has a non-fired modulus of rupture ranging from 250 psi to 700 psi after fully hardened. 